1. Field of the Invention
The present invention relates to semiconductor photodetector devices, and more particularly to semiconductor photodetector devices having an avalanche effect, and a method of manufacturing them.
2. Description of the Disclosure
A so-called avalanche photodiode (hereinafter abbreviated as an APD), wherein carriers are generated by irradiating light onto a pn junction applied with a reverse bias voltage, and are avalanche-multiplied by impact ionization caused by the carriers, is featured by its remarkable internal multiplication effect, high response characteristic, etc. Among various types of APDs, an InP-based APD whose light absorption layer is formed of either InGaAs or InGaAsP and whose carrier-multiplication layer is formed of InP is particularly advantageous, in light of its high quantum efficiency and low noise characteristic. This type of InP-based APD is useful as a semiconductor photodetector device for detecting waves in the 1 .mu.m band, and constant efforts are being made to develop an improved, practical InP-based APD.
FIG. 1 illustrates the conventional APD disclosed in Published Unexamined Japanese Patent Application No. 57-198667. This conventional APD is manufactured by executing a crystal growing step twice. The manufacturing process will be briefly mentioned. First of all, n.sup.- -type InP buffer layer 52, n.sup.- -type InGaAs light absorption layer 53, n-type InGaAsP buffer layer 54, and n.sup.- -type InP layer 55 are formed on n-type InP substrate 51 by crystal growth in the order mentioned (the first crystal growing step). Next, Si ions serving as n-type impurities are implanted into a selected portion of n.sup.- -type InP layer 55, and the resultant structure is annealed, to thereby form n-type InP region 56. After n.sup.- -type InP layer 57 is formed on the resultant structure by crystal growth (the second crystal growing step), p-type region 59 serving as a guard ring is formed by the implantation of Be ions and subsequent annealing. Thereafter, p.sup.+ -type region 58 serving as a light-receiving region (which includes a pn junction between n-type and p-type semiconductors and its neighboring portions) is formed by the diffusion of Cd. Finally, SiN film 60 serving as a reflection-preventing film, p-type electrode 61, and n-type electrode 62 are formed.
In general, a guard ring is formed for the purpose of preventing an edge breakdown from occurring in regions which are located around a light-receiving region and in which the radius of curvature of a pn junction is short. For this purpose, the guard ring is made to have a graded junction, whereas the light-receiving region is made to have an abrupt junction. In the structure illustrated in FIG. 1, the guard ring fulfills its function effectively since n-type low concentration layers 57 and 55 are located under the guard ring so as to decrease the maximum electric field and thereby suppress the breakdown in the guard ring region. On the other hand, an n-type high concentration region 56 is located under the light-receiving region so as to increase the maximum electric field in the light-receiving region.
To manufacture the above-mentioned structure, crystals of InP have to be grown after the selective ion-implantation of n-type impurities and the succeeding annealing. Normally, this crystal growth has been performed by using a meltback technique in the liquid phase epitaxial growth process, so as to provide a satisfactory interface and a crystalline region. In recent years, however, there is a tendency to manufacture compound semiconductor devices by use of a vapor phase epitaxial growth process, such as an MOCVD process, which is superior to a liquid phase epitaxial growth process in terms of the controllability of carrier concentration and film thickness. With this tendency, more APDs have come to be manufactured by use of the vapor phase epitaxial growth process, as long as their manufacture requires only one crystal growing step. In the case of the manufacture of super high-speed APDs, the vapor phase epitaxial growth process is indispensable.
However, if a crystal growing step is required twice, as in the manufacture of the above-mentioned structure, and if the second crystal growing step is performed by use of the vapor phase expitaxial growth process, then distortion of the lattice and precipitation of impurities at the interface will be more marked than in the case where the meltback technique is used in the liquid phase epitaxial growth process. And in the case of the above-mentioned structure, the interface formed by the second epitaxial growth step will be located in the n-side region of the p.sup.+ n junction, as may be understood from FIG. 1. Since, therefore, a high electric field is supplied at the interface which is located n-side region during APD operation, a dark current may increase and local breakdown may occur. In addition to these problems, the above-mentioned structure has problems in that its guard ring is inevitably formed in the region formed by the second crystal growing step (which region does not have a good crystalline condition). These problems may also occur where the second crystal growing step is performed by use of the liquid phase growth process.